Changed button text from 'Submit' to 'Next'

'Submit' breaks the guidance given on the buttons page:
"Don’t use technical terms - eg. use ‘delete’, not ‘form reset’"
Added 'submit' under technical terms to avoid.
Added a line under 'common actions'
Secondary example 'Next step' changed to 'Save'

To avoid contradicting with the guidance which follows it:
"They [Secondary actions] don’t move users to the next step"
Re-sorted advice to give preferred advice first.
Grouped advice on 'Sign up' with 'Sign in'
‘Create an account’ instead of ‘Register’
Added section: pre-checked radios and checkboxes

1. I don't know if my formatting is hierarchically correct here.
2. This advice seems sensible to me – does it to you? Wider discussion needed.
Button positioning. Tim, please see description.

I've added a form field to this example to show button alignment, but have been unable to test that this displays correctly.

Also, see this research on the positioning of secondary actions: http://www.lukew.com/ff/entry.asp?1502 
Link to Front-end Toolkit
Merge pull request #79 from alphagov/nicepaul

nicepaul – various form changes